It is difficult to produce a conformal coating of complex 3D surfaces. Especially challenging is deposition of a biologically active layer, such as cell-laden collagen, that cannot be sprayed (too much shear stress will damage cells) or brushed across surface (the collagen layer will not coat the surface uniformly, leaving uncovered areas behind protrusions due to shadow effect, and this type of coating is too labor-intensive) and other techniques such as dip-coating do not work due to non-uniformity of the resulting film. Spin-coating cannot be applied due to viscosity of collagen material as well as un-coated zones behind the protrusions (same type of “shadow” effect as for brush-coating). Additionally, other techniques cannot guarantee a smooth surface with uniform layer thickness.
It is possible to use 3D printing (Kim, T. G., et al., 2010 “Microstructured scaffold coated with hydroxyapatite/collagen nanocomposite multilayer for enhanced osteogenic induction of human mesenchymal stem cells” J Mater Chem 20: 8927-8933), but this approach only works if coating is deposited at the same time as a material is built, e.g., when one layer of matrix is deposited, another layer of collagen is dispensed, then matrix is deposited again, then collagen again. In other words, use of 3D printing for surface coverage is not effective for two reasons: (1) in order to have complete coverage, the lines deposited by 3D printer should overlap, requiring significant time, and (2) in case of complex geometries, the 3D structures will not allow a printer head to get close enough to coat all the surface (it might work for flat surface with very small protrusions, but not for large protrusions, or curved surfaces).
It is possible to produce a negative mold of a surface, coat it with a biolayer of interest and press that mold to a 3D surface, but it is difficult to control uniformity and thickness of the layer. Additionally, this technique requires the production of molds for each surface to be coated.